Installing dryer into sub-panel - Nearly full service

First time posting here.
I want to switch from a gas to electric dryer. My main box is full complete with a hot tub hook up. I have a sub-box with 8 breaker positions, 6 of which are filled. I have the appropriate wire run from the dryer to the main box (the previous owner ran the electrical even though it was a gas dryer). I just wonder if adding a dryer to the subpanel would overload the system. It is 100amp service. As I said, it has a hot tub set up on it (with outside hot tub panel) and service ran to the garage from the subpanel.

I'd simply like to know if I could fill those last two switches, what size of breakers I would need and whether it would be safe to do so. The alternative is to increase to 200amp service; however, that is a large and expensive job which I don't wish to do at this point.
Thanks in advance.

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Installing dryer into sub-panel - Nearly full service

Hello, I notice you used the word "service" which means a specific thing in this context -- the utility service to your building. So your main panel has a 100A service, is that correct?

What size breaker is supplying the sub-panel? What loads are connected to the 6 existing circuits in it?

Do you have any other large electrical loads? I'm guessing your heat is gas-fired; what about your water heater, range/oven? What is the draw for your hot tub when it is running? Are you willing to live with shutting off your hot tub in order to use the electric clothes dryer?

What is the current draw for the clothes dryer? Some can be installed on a 30A 240V circuit, others need 50A!

There are a lot of unknowns here. You are right to be concerned about this and it is good you are researching it before you bought the electric dryer.

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Installing dryer into sub-panel - Nearly full service

That really depends on the size of the house and what you are running in it. A load calculation is the only way to know for sure.

But I do know this. That 100 amp service was never intended to run your refridgerator and range and microwave and dishwasher and dryer and washing machine and furnace and garage and we need an AC in the window and a fish tank and TV and sound system and computer and ......
I forgot the hot tub!